I agree with blocking trades in 2p (as it is very rare that a a mutually beneficial trade can be achieved in 2p), more often that not it is used by the losing player to try and con the other player, or simply to annoy and distract them.
I'm sure some will disagree with this however.

I fully agree with blocking the auctions of properties already owned by any player during ec, as yes this is generally unfair and used to annoy or ruin games.

I'm thinking about this, not sure about the per one card thing.
Although the values being low would probably not cause any problems.

Might be worth you speaking to crabbs, he's very clued up on trading and does a lot of trading in games. He also does a lot of thinking outside of the box on trades and comes up with some that a lot of people wouldn't think of.

I like the fact that some are being changed. Espicially the yellow, it shall be intresting to see how they are treated since they are usually neglected in 2p.

I think the following should remain as they currently are

orange - 100
brown - 200
cyan - 300
blue - 400

My favourite part is trying to find the best bargin property. It should also be the case that players who buy way to many properties can be punished buy giving away cheap properties elsewhere. Im personally not a big buyer of orange and browns at their current opening price and give them away at 160 sometimes in 2p (which someone might take advantage of haha).

In my oponion keep them as they currently are, otherwise the oranges and browns might never get used in most games.
Other then that i really like the other changes _________________

1 - OK
2 - Personally I don't agree. Sometimes players refuse to trade with you, but when you force them to decide in an auction, they pay up.
3 - I think I understand. This is to avoid players tricking each other by making it look like a full property trade when the result is useless as you still don't have a full set. It's a good idea.

As Krabs has presented, limits or "set prices" for sets or properties really can't be made as the whole game is based on the situations that occur during it, making no two games alike. Best these "limits" do is to serve as suggestions.

As i described above, we can prevent cheating when two players have 2 sets for both. And first player receives set, but does not give the set for the second (as i described above).
I hope that in time we will gather cheat statistic and can detect typical patterns. After that we could program algorithm against these cases. But i am not sure that we will detect such patterns, because this game has so many variants.